sakuya ha kyuuni teidensite, heya ga kuraku narimasita.

That is a very common question.

Yes, time expressions like 昨夜, 今日, 明日, and 去年 often appear with no particle at all:

• 昨夜、急に停電して、部屋が暗くなりました。

Adding は gives 昨夜 a topical or contrastive feeling:

• 昨夜は = as for last night / last night, ...

So in this sentence, 昨夜は sets the scene and slightly highlights last night as the time frame being talked about. It is natural, but 昨夜 without は would also be possible.

停電 is a noun meaning power outage / blackout, but it is also a suru-verb noun.

So you can say:

• 停電する = to have a power outage occur / for the power to go out
• 停電して = the te-form of 停電する

In this sentence, 停電して comes from 停電する.

Because the sentence is connecting two events:

1. a power outage happened
2. the room became dark

The te-form is often used to link actions or events in sequence, and it can also suggest a cause/result relationship.

So:

• 停電して、部屋が暗くなりました。

means something like:

• There was a power outage, and the room became dark.
• The power went out, so the room became dark.

Only the final verb in the sentence, なりました, carries the past/polite ending here.

Most naturally, 急に modifies the event right after it:

• 急に停電して = the power suddenly went out

But in practice, it colors the whole situation. Because the blackout caused the room to become dark, English could also feel it as:

• Suddenly, there was a power outage, and the room became dark.

So grammatically it is closest to 停電して, but semantically it affects the whole scene.

Here, が marks 部屋 as the subject of 暗くなりました:

• 部屋が暗くなりました = the room became dark

Using が is a neutral way to present what happened.

If you said:

• 部屋は暗くなりました

that would make the room the topic, possibly with a contrastive nuance, such as the room became dark, but maybe somewhere else did not.

So が is very natural here because the sentence is simply describing the result of the blackout.

Because with い-adjectives, you change -い to -く before なる.

Pattern:

• 明るい → 明るくなる = become bright
• 暗い → 暗くなる = become dark
• 寒い → 寒くなる = become cold

So:

• 暗い = dark
• 暗くなる = to become dark
• 暗くなりました = became dark

This is the standard grammar for expressing a change of state with an い-adjective.

なる means to become.

So:

• 部屋が暗い = the room is dark
• 部屋が暗くなる = the room becomes dark

The sentence is describing a change, not just a state. The room was not dark before, and then it became dark after the blackout. That is why なる is the natural choice.

Japanese often leaves the subject unstated when it is obvious or unnecessary.

With 停電する, the expression itself describes the event of a power outage occurring. You do not need to name a subject like electricity or the building.

So:

• 急に停電して naturally means the power suddenly went out / there was suddenly a blackout

This is one of those cases where English often wants a subject, but Japanese does not need one.

In connected Japanese clauses, the final predicate usually carries the tense and politeness for the sentence.

So in:

• 停電して、部屋が暗くなりました。

the first part is in te-form to connect it, and the second part ends the sentence in past polite form.

A plain-form version would be:

• 昨夜は急に停電して、部屋が暗くなった。

A polite version is:

• 昨夜は急に停電して、部屋が暗くなりました。

This is normal Japanese sentence structure.

Yes, depending on context, that can sound very natural in English.

Japanese says:

• 停電して = there was a power outage / the power went out
• 部屋が暗くなりました = the room became dark

In English, a smooth translation might be:

• Last night, the power suddenly went out, and the room became dark.
• Last night, the lights suddenly went out, and the room went dark.

The Japanese is specifically about a power outage, but a natural English translation may shift wording slightly.

Yes, both can mean suddenly, but there is a slight difference in feel.

• 急に is extremely common in everyday speech and writing.
• 突然 can sound a little more formal, literary, or dramatic.

So:

• 昨夜は急に停電して... = very natural, everyday
• 昨夜は突然停電して... = also correct, but a bit more forceful in tone

For a basic conversational sentence like this, 急に is a very natural choice.